Apparatus for manufacturing diamonds

ABSTRACT

A machine is disclosed for converting graphite to diamond which includes a cylindrical reaction chamber and a reaction vessel positioned in the reaction chamber for holding the charge of graphite to be converted to diamond. A piston having a fixed length and fixed diameter is positioned in the reaction chamber with its first end contacting the reaction vessel and its second end extending out of the reaction chamber. Means are provided for applying a force to the second end of the piston, thereby generating high pressure within the reaction chamber. Further means are provided for maintaining the ratio of the unsupported length to the diameter of the piston at a value less than the ratio of the fixed length to the fixed diameter of the piston.

United States Patent Kennedy vDec. 5, 1972 [s41 APPARATUS FORMANUFACTURING 2,992,900 7/1961 Bovenboeck ..19/mc. 26 DIAMONDS 3,082,4773/1963 Custers et al... ..l8/DIG. 26 [72] Inventor: George C. y, Los g3,407,445 ill/1968 Strong ..18/DlG. 26

Cahf' Primary Examiner-J. Howard Flint, Jr. [73] Assignee: Teledyne,Inc., Los Angeles, Calif. Attorney-Ronald W. Reagin, Stephen L. King an[22] Filed: Aug. 13,1970 Kenneth Mate [21] Appl. No.: 63,356 I [57]ABSTRACT A machine is disclosed for converting graphite to 52 us. Cl..425/77 diamond which includes a cylindrical reaction [51] Int. Cl...B30br1l/32 chamber'and a reaction Vessel pfl'sitionfid in the [58]Field of Search ..425/77; l8/DIG. 26 tion chamber for holding the chargeof graphite to be converted to diamond. A piston having a fixed length 5References Cited and fixed diameter is positioned in the reactionchamber with its first end contacting the reaction ves- UNIT ED STATESPATENTS sel and its second end extending out of the reaction chamber.Means are provided for applying a force to s gg the second end of thepiston, thereby generating high I a pressure within the reactionchamber. Further means 3,25 Sturm are provided for maintaining the ratioof the unsup 3313871 4/1967 vgel et 26 ported length to the diameter ofthe piston at a value 3,555,597 1/1971 Meadows ....18/DIG.26 less thanthe ratio of the fixed length to the fixed 3,067,465 12/1962Giard1nietal.... ....18/DIG. 26 diameter of the piston I 2 3,423,7941/1969 Wilson ....l8/DIG. 26 t 7 2,941,250 Hall 4... ..18/DIG. 26 10Claims, 7 Drawing Figures PATENTEDnEc 5 I972 SHEET 1 0F 3 TEMPERATURE INC Zflig. 2 PRIOR ART K a 26 -\i GEORGE C. KENNEDY INVENTOR.

ATTORNEY PATENTEDMI: 5 I972 SHEET 2 BF 3 Fig.3

GEORGE C. KENNEDY INVENTOR.

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ATTORNEY PATENTEDnEc 5 I972 SHEET 3 0F 3 m w w w mmqmo zx 2 Ihozmmhm 02Im3m0 GEORGE c. KENNEDY INVENTOR. Ea/M5,;

ATTORNEY TEMPERATURE IN C' APPARATUS FOR MANUFACTURING DIAMONDSBACKGROUND OF THE INVENTION I a particular crystal configuration. It haslong been known that if other inexpensive forms of carbon, such asgraphite, are subjected to sufficient temperature and pressure, it canbe converted into diamond. For example, Moisson first produced very tinydiamonds in 1890. This was done by dissolving graphite in a suitablesolvent at a high temperaturesuch that the solvent was in the liquidphase and then chilling the liquid solution. The liquid solvent wasconverted back into the solid phase and set up extremely high internalpressures in the solvent-graphite droplet which, in combination with thehigh temperature at which the solvent and graphite had previously been,resulted in the graphite being converted into diamond. Of course, thesemethods were not susceptible to any degree of'control and it was .not aneconomic means of. manufacturing diamond. This was considered to benothing but a laboratory curiosity for many years. 1

In more recent years the problem of manufacturing diamonds has receiveda. great deal of attention from investigators in the field and largesums of money have been spent by competent organizations to perfectcommercially feasible apparatus and methods'for manufacturing diamond.Particularly, the General Electric Company has made a major effort inthisfield and has achieved a fair degree of success in thediamond makingfield. The basic results of General Electrics efforts in this field areshown in U.S. Pat. Nos. 2,941,248 Hall, which teaches a high temperaturehigh pressure apparatus which is particularly designed tomanufacturediamond, and 2,947,610 Hall et al., which teaches and claims a methodfor making diamond. The method patent discloses that in order tomanufacture diamond, the graphite must be mixed with a catalyst materialselected from a group of catalysts disclosed therein. The mixture mustthen be subjected to a pressure of at least 7 kilobars and heated to atemperature of from 1200 to 2000 C. The high temperature high pressureapparatus patent discloses and claims a particular apparatus capable ofdeveloping the high temperatures and high pressures mentioned whichGeneral Electric felt was necessary to manufacture diamond.

In a related General Electric Patent, US. Pat. No. 2,947,609 Strong, itis disclosed that diamonds can be manufactured in the Hall apparatus atpressures as low as 50 kilobars if particular metalalloys disclosed byStrong are used as catalysts.

The General Electric method and apparatus unquestionably manufactured,diamond. However, the diamonds so manufactured were of quite small size,usually less than H 100th of a car at,'and the diamonds almost alwayscontained inclusions, or small impurities within the diamonds which werebits of the catalyst metal.

There are two basic reasonswhy the prior art method of making diamondshas resulted in, at best, small stones of poor quality. The first ofthese reasons is because of a basic misunderstanding of what wasoccurring in the high temperature high pressure apparatus during thediamond conversion. The second reason is because of inherent limitationsin the apparatus which was being used.

The prior art investigators made two fundamental mistakes in theirtheoreticalanalysis of the manufacture of diamonds. The first mistakewas the error, at times quite extreme, in measuringthe pressure withinthe high pressure apparatus. Because of the inherent nature of theapparatus, discussed :in more detail below, as the applied force 'on theapparatus is increased, a higher and higher proportion of'theforceappears as load on the chamber walls of the apparatus rather than ashigherpressure within the reaction chamber. A point is soon reachedwhere practically all incremental increases in force appear as. chamberwall load and none of the increase in force is reflected as increasedpres- V sure within the reaction chamber. Thus, when the investigatorsthought they were making diamonds at a pressure of 75 kilobars, in factthe pressure in the reaction chamber was probably closer to. 52kilobars. Since the pressures were being measured so poorly, obviouslythe investigatorscould not maintain the precise controls needed toobtain meaningful date about the nature of thereaction and to determinewhich precise conditions produce the best results. As a result, in orderto manufacture diamond the apparatus was operated at loads substantiallyhigher than was really of a few large crystals from the same chargeinstead.

.The second error in the theoretical analysis was in the description ofthe catalysts. Allof the materials described as catalysts in the abovementioned Hall and Strong patents and in other General Electric patentsand publications reporting their findings in this area have one factorin common. They are all good solvents of carbon when they are in theirliquid phases. In fact, these catalysts are not catalysts in thereactions at all, but are merely solvents which take the graphite intosolution while diamond is beingpr'ecipitated from solution duringthe'high temperature high pressure portion of the reaction. Any materialin which carbon is sufficiently soluble can be used instead of thevarious metals or alloys disclosed in the above mentioned Hall andStrong patents. The lower temperature and pressure extremities at whichgraphite can be converted to diamond then becomes strictly a function ofthe melting point of the solvent at high pressure. There is a large bodyof data available as to the degree of solubility of carbon in varioussolvents. Indeed, the solubility of carbon in all of General Electricscatalysts was disclosed by Moisson many years ago. As was mentionedabove, since again the prior art investigators did not have a goodunderstanding of the reactions that were, in fact, occurring, they wereunable to develop the precise meaningful data which might have enabledthem to develop controllable methods for producing suitable diamondshaving desired physical properties such as size and impurity limits.

The second reason that the prior art was unable to reproduce diamonds ona satisfactory basis lies in the inherent nature of the apparatus used.This apparatus is known to those skilled in the art as the belt"apparatus. As'is described in detail below, the apparatus simplyconsists of a tapered punch which fits into a correspondingly tapereddie. A gasket material is placed between the punch and the die andsuitable force is placed on the punch, which results in the gasketmaterial being extruded from between the punch and the die. This has twoeffects, the first being to insure a good seal between the punch and thedie and the second being to reduce the volume of the reaction chamberwithin the die and to transmit high pressures into the reaction chamberso that the graphite therein is subjected to suitable conditions tocause it to be converted to diamond. Because of the inherent nature ofthe tapered punch and correspondingly tapered die, the punch is capableof only a limited excursion into the die and thus the reaction chamberis inherently capable of only relatively slight changes in its volume.The difficulty with this is that diamond is substantially denser thangraphite and thus when the graphite beginsits conversion to diamond,there is an abrupt decrease in the volume of the charge. Since the punchcannot traverse far enough into the die, this results in an abrupt dropin pressure on the charge when the diamond conversion process actuallybeings. The belt apparatus is thus incapable of maintaining thenecessary high pressures for more than a brief instant of time.

The' above mentioned Hall patent on high temperature high pressureapparatus does disclose the use of a cylindrical punch and correspondingdie but indicates that such apparatus will fracture before it willdevelop the necessary high pressures for diamond manufacture. Thestrongest materials available then and at the present time, cementedtungston carbide, rupture under pressures in the order of 47 kilobarsand thus the prior art teaches that such apparatus cannot be used forthe manufacture of diamonds.

OBJECTS OF THE INVENTION to diamond in which larger diamonds can bemade. It is still another object of this present invention to provide animproved apparatus for converting graphite to diamond in which thediamonds so produced are free from inclusions.

BRIEF DESCRIPTION OF THE INVENTION Briefly stated, and in accordancewith the presently preferred embodiment of the invention, a machine forconverting graphite to diamond is provided which includes a cylindricalreaction chamber and a reaction vessel positioned in the chamber forholding the charge of graphite to be converted to diamond. A pistonhaving a fixed length and fixed diameter is positioned in BRIEFDESCRIPTION OF THE DRAWINGS A complete understanding of the invention,together with an appreciation of all objects and advantages thereof, maybe seen by reference to the attached drawings, in which:

FIG. 1 shows the pressure-temperature phase diagram for carbon andillustrates the conditions under which carbon can exist in the diamondstate and in the graphite state;

FIG. 2 shows a schematic representation of the prior art apparatus forconverting graphite to diamond, and illustrates the disadvantagesassociated therewith;

FIG. 3 is a schematic representation of high-temperature high-pressureapparatus for making diamonds in accordance with the present invention;

FIG. 4 is a cross-sectional view of a high-pressure high-temperaturereaction chamber of the apparatus of FIG. 3 and particularly illustratesmany of the features of the present invention;

FIG. 5 is a graphic representation of the crushing strength of thepiston of FIGS. 3 and 4 as a function of the ratio of the length of thediameter of the piston;

FIG. 6 is a cross-sectional view of the high-temperature high-pressurereaction vessel of FIGS. 3 and 4; and

FIG. 7 shows another phase diagram for carbon and illustrates one methodunder which graphite may be converted to diamond in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS Before considering in detail thedeficiencies of the prior art and the specific manner in whichthepresent invention overcomes these deficiencies to provide larger andhigher quality man-made diamonds, it will be useful to consider in thegeneric sense what operations must be performed upon graphite to convertit into diamond. As is well known, both graphite and diamond are carbon,but carbon whose atoms are arranged in a different crystalline form. Atatmospheric temperatures and pressures, graphite exists in its stablestate and diamond exists in its metastable state. Further, at extremelyhigh temperatures and pressures, carbon can exist in the stable state inonly one or the other form, dependingupon the particular temperature andpressure. Generally speaking, for a given high temperature, at highpressures diamond is the stable state of carbon and at lower pressuresgraphite is the stable state. Conversely, for a given high pressure, athigher temperatures graphite is the stable state of carbon and at lowertemperatures, diamond is the stable state.

It is noted that when the term graphite is used in the specification andclaims, it is not necessarily intended to be construed strictly, but isintended to mean any form of carbon other than diamond, regardless ofits crystalline form.

FIG. 1 shows graphically the pressure-temperature phase diagram ofcarbon, with pressure shown in computed and also measuredexperimentally. For example, the location of the line is reported by R.Berman and F. Simon in Zeitschrift fur Electrochemic, 59,333 (1955).However, aswas mentioned before, some of the experimental results whichhad beenreported have erred considerably on the high-pressureside-because of the reasons discussed in more detail in connection withFIG. 2 below. I 1

In view of the known location of the equilibrium line 1, it would seemthat the conversion of graphite to diamond wouldbe a simple enoughprocess. All that is necessary is to subject the graphite to acombination of pressure and temperature lying above the equilibrium line1, and the conversion would be naturally effected. However, in practiceobviously the conversion has not been that simple. First the pressuresand temperatures shown in FIG. 1 which lie above the equilibrium line 1are extremely high pressures and temperaturesand it is difficult todesign and operate suitable apparatus for containing reactions occurringat these temperatures and pressures. Next, the mere measurement ofpressures and temperatures of these great magnitudes is quite difficult,asis discussed in considerably more detail in connection with FIG. 2below, so it is difficult to know when the proper conditionsfor thediamond conversion are being obtained. Third, even if suitable apparatusfor effecting the conversion is designed and even if the conditions arebeing accurately monitored so that desired pressures and temperaturescan be achieved, it is not sufficient just to apply, any combination oftemperature and pressure lying above equilibri um line 1. In order toachieve optimum conversionof graphite into large diamond crystals freeof inclusions, or small particles of foreign matter, it is necessary tosubject the graphite to the the proper combination of pressure andtemperature. If excess pressures and temperatures are applied, theresult is the conversion of graphite to diamond, true enough, but whatis obtained is hundreds of very tiny diamond crystals containing manyinclusions, which is obviously not as desirable as converting the samecharge of graphite into a much smaller number of much larger diamondcrystals which are free of such inclusions. Fourth, experience has shownthat it is not sufficient just to subject pure gra- Q prior artinvestigators astray. They assumed that since some carrier material hadto be present which did not itself appear to enter into the reaction,this carrier material was a catalyst. The prior art reports many suchcatalysts. For example, in U.S. Pat. No. 2,947,610

6 I-Iallet al., it is reportedthat asuitable catalyst material forconverting graphite to diamond can be selected from the class consistingof iron, cobalt, nickel, rhodium, ruthenium, palladium, osmium,iridium,chromium, tantalum and manganese, In U.S. Pat. No. 2,947,609Strong, it is reported that the catalystcan be an alloy whose componentsare selected from the same group of metals plus platinum. y I In theinvestigations which led to the present invention, it was observed thatall of the so-called catalysts have one physical property in common.This is that all of these materials have been known for 'many years tobe good solvents of carbon at temperatures sufficiently high for thematerial to be its liquid phase. Further investigation showed thatdiamond conversion could be effected in any material'which woulddissolve carbon well when it was in its liquid phase and that conversioncould not be effected in those materials which were not good solvents ofcarbon. From these observations it is concluded that conversion fromgraphite to diamond can be effected if the following conditionsar'e'met: (1

the graphite is first mixed with a suitable quantity of material which,when the material is in its liquid phase,

it is agood solvent of carbon and (2) the mixture is then subjected totemperature and pressure conditions at which the material is in itsliquid phase and which lies above the equilibrium line of FIG. 1. Whenthese conditions are considered, it is seen that the so-called catalystsreported in the prior art have no catalytic effect whatsoever on theconversion of graphite into diamond, but are instead merely suitablesolvents which take graphite into solution, and which precipitatediamondout of solution so that the conversion can be effected.

Now referring still to FIG. 1, the line 2 represents the plot of themelting point of a material asa function of pressure and temperature.Line 2 can be termed the solid-liquid phase boundary line. In thisparticular'case, the phase boundary line 2 represents the boundarybetween the liquid and solid phase of a nickel-iron alloy having percentnickel and 50 percent iron. On the left-hand side of phase boundary line2, the alloy is in its solid phase and on the right-hand side of phaseboundary line 2, thealloy is in its liquid phase. This particular alloyis known to be a good solvent of carbon when it is in its liquid phase,and thus this alloy is, under the conditions described above, a goodsolvent for use in a graphite to diamond conversion. The point 3, whichis the intersection of equilibrium line 1 and phase boundary line 2,represents the lowest temperature and pressure at which graphite can beconverted into diamond when mixed with this particular solvent.

7 The shaded region 4 bounded by the section 5+3 of FIG. 2 shows aschematic representation of the type of prior art high pressure hightemperature apparatus known as the belt apparatus,'such as is disclosedin the above mentioned U.S. Pat. No. 2,947,610 Hall et al. and U.S. Pat.No. 2,941,248 Hall. As shown therein, the apparatus includes two punches12 and- 14, each positioned on opposite sides of an opening in anannular die 16. Each of the punches 12 and 14 includes a respectivetapered portion 18 and 20 which tapered portions enter the reactionchamber 22 of die 16 when punches 12 and 14 aremoved towards die 16.Reaction chamber 22 is bounded by tapered surfaces 24 and 26 which arecomplimentary to the surfaces of the tapered portions 18 and 20 ofpunches 12 and 14, respectively. Two gaskets 28 and'30 are providedwhich fit between the tapered punch portions 18 and 20and theirrespective tapered surfaces 24 and 26 in die 16. The gaskets 28 and 30are usually constructed from a suitable ceramic material such aspyrophyllite.

In operation, the graphite to be converted to diamond is placed in areaction vessel 32, shown schematically, which is in turn placed inreaction chamber 22 of die 16. The gaskets. 28 and 30 are placed aroundreaction vessel 32 and the dies 12 and 14 are brought into contact withthe gaskets 28 and 30. Reaction vessel is then heated electrically to asuitable temperature and suitable'forces are applied to punches 12 andl4. to

.raise the pressureon reaction vessel 32 to the proper conditions atwhich diamond conversion occurs.

The forces applied onpunches l2 and 14 are applied directly-onto theends of reaction vessel 32 and onto the inner surfaces of gaskets-28 and30. As these forces are increased, the tapered portions 18 and 20 ofpunches l2 and 14 make limited excursions in reaction chamber 22. Asthis occurs, the thickness of gaskets 28 and 30 becomes thinner andthinner, with the gasket material being extruded out of reaction chamber22. The gaskets 28 and 30 thus provide the dual function of maintaininga suitable pressure seal within the reaction chamber 22 while at thesame time allowing the tapered portions 18 and 20 of punches 12 and 14to make this limited excursion into reaction chamber 22. However, theamount of thisexcursion is obviously limited to an amount at which thethickness of .the gaskets 28 and 30 approaches zero. In practice, it hasbeen found that the minimum thickness to which the gaskets 28 and 30 canbe extruded is about 0.05 inches.

There are two severe drawbacks to the belt apparatus just described.Both of these drawbacks are inherent in the tapered shape of the punchand the die. The first problem lies in accurately determining what isthe actual pressure appearing in reaction chamber 22. It is readilyappreciated that part of the forces applied through punches l2 and 14appears on the tapered innersurface of gaskets 28 and 30 and that only aportion of the applied force appears as pressure in reaction chamber 22.As the forces are increased and the gaskets 28 and 30 are extrudedthinner and thinner, a point is soon reached where practically allincremental forces applied through punches l2 and 14 appear on 8sufficient forces to dies 12 and 14 to create what they thought was apressure of perhaps kilobars within the reaction chamber 22, the actualpressure was probably more in the order of 52 kilobars.

The second inherent problem with the belt apparatus of FIG. 2 lies inthe inherent limited excursion of punches 12 and 14 into die 16. As wasdiscussed above, and as is readily obvious from examination of FIG. 2,the length of excursion of punches 12 and 14 into die 16 is limited tobeing a fraction of the thickness of gaskets 28 and 30. Since the volumeof reaction chamber 22 is very nearly a linear function of the positionof the end of the punches, reaction chamber 22 can undergo only alimited incremental change in volume. This might not appear to be toosignificant a problem, until one considers that there is a significantdifference in the density of diamond and the density of graphite. Thedensity of diamond is about 3.4 while the density of graphite is only2.23. Thus, when the proper temperature and pressure conditions arepresent and the graphite begins converting into diamond, there is anabrupt decrease in the volume of the material in reaction chamber 22. Inorder to maintain the necessary high pressure for any period of time,the punches 12 and 14 must be able to traverse further into reactionchamber 22 to compensate for this drop in volume or else the drop involume will be immediately reflected as an abrupt drop in pressure inreaction chamber '22. As is seen, the volume of reaction chamber 22 inthe belt apparatus is inherently incapable of any significant change involume, and thus the apparatus can maintain the necessary high pressureon a reactionvessel 32 loaded with graphite only until a sufficientquantity of the graphite has been converted into diamond to cause thevolume to decrease to a point at which the punches 12 and 14 cantraverse no further into reaction chamber 22. In practice, it has beenobserved that once conversion begins, the pressure can be maintained inthe diamond stable field only for a few seconds.

What would frequently occur when belt apparatus of the type shown inFIG. 2 was used was that the operators would apply toomuch force to thepunches, since they did not know with any degree of accuracy at all whatresultant pressures appeared in the reaction chamber, and this wouldresult in the graphite charge in the reaction vessel being subjected totemperatures and pressures far above the equilibrium line 1 of FIG. 1 toa point well into the diamond growing region 4 of FIG. 1. This suddenplunge into the diamond growing region would cause many small diamondparticles to be formed, instead of only a few larger diamond particles.

Also, because of the above mentioned change in density, the pressure inthe apparatus would quickly drop back to a point below equilibrium line1 and none of the small diamond particles formed could be subjected tothe necessary growth conditions for a sufficient period of time to growinto sizable crystals themselves.

FIG. 3 shows a schematic representation of high pressure apparatus 40which may be used in accordance with the present invention to convertgraphite to diamond. The apparatus 40 includes a high pressure plate 42which includes a cylindrical central cavity 44 which forms the reactionchamber in'which the diamond conversion occurs. This reaction chamber 44corresponds to the reaction chamber 22 of the belt apparatus of FIG. 2.A cylindrical shaped piston 46 fits into reaction chamber 44. As isdescribed in more detail below, the necessary high pressure isgeneratedinreaction chamber 44 by advancingpiston 46 into reaction chamber44. I

High pressure plate 42 is bounded on its upper and lower ends by upperend load plate 48 and lower end load plate 50 respectively. These endload plates support the ends .of high pressure plate 42 to prevent itfrom rupturing when it is under extreme pressure within reaction chamber44. Inaddition, upper end load plate 48 serves to seal the top end ofreaction chamber 44. Lower end load plate 50 includes a central openingthrough'which piston 46 projects into reaction chamber 44. I i v Thenecessary forces to move piston 46 into reaction chamber 44, therebygenerating the high pressure within reaction chamber 44, are applied topiston 46 through piston pusher 52. The forces are generated in ahydraulic ram 54, shown schematically- The force cycle is closed bypositioning member 56 below hydraulic ram 54 and member 58 above upperend load plate 48 and tying members 56 and 58 together with tie rods 60.Thus when hydraulic ram 54 is actuated, piston 46 advances into intoreaction chamber 44 and upper end load plate 48 is held tightly over theend of reaction chamber 44, thereby generating high pressures therein.

When extremely high pressures are generated in reaction chamber 44,forces are created in high pressure plate 42 which might cause it toruptureout its ends. To prevent this, a second hydraulic ram 62 isprovided having one end positioned above member 58. Another member 64 isprovided above the other end of hydraulic ram 62. Yet another member, 66is provided beneath lower end load plate 50 and members 64 and 66 aretied together through tie rod- 68. It is noted that member 66 includes acentral opening through which piston pusher 52 contacts piston 46 andadditional openings through which tie rods 60 interconnect members 56and 58. When hydraulic ram 62 is actuated,

high pressures are thus placed on the ends of high pressure plate 42through end load plates 48 and 50to counteract the stresses generatedinternally in high pressure plate 42 when high pressures are generatedin reaction chamber 44.

It is noted that high pressure apparatus of the general type shown inFIG. 3 is discussed by Kennedy and LaMori in an article entitled SomeFixed points on the High Pressure Scale appearing in Progress in-VeryHigh Pressure Research, Bundy, Hibbard & Strong, Ed., J. Wiley & Sons,1961, and in an article by Kennedy, Haygarth and Getting entitledDetermination of the Pressure of Barium I-Il Transition with SingleStage Piston Cylinder Apparatus appearing in The Journal of AppliedPhysics, Volume 38, No. 12,

4557-4564, Nov. 1967,

FIG.4 shows a cross sectional viewof an enlarge ment of the detail 70 ofFIG. 3 and illustrates certain features of the present invention which,for clarity, are

[ material capable of withstanding the highest possible pressures-Of thepresently knownrmaterials, cemented tungsten carbide is preferredbecause of its outstanding ability to withstand compressive forces.However, tungsten carbide is quite brittle and so it is necessary tosurround it with the support rings 74,76 and 78, which may be made ofhigh-grade steeLSupport rings 74,76 and 78 are shrunk onto pressurevessel 72 and to each other to provide the tightest interference fit.Also, it has been found desirable to provide a slight angulartaperbetween the various members, as shown in FIG;

construct piston 46 from the material capable of withstanding thehighest compressive forces, such as tungsten carbide. In operation,piston 46. is-advanced into reaction chamber 44 by piston pusher 52, aswas described in detail in FIG. 3 above, until onefend of piston 46contacts the end of a reaction vessel 80, to provide high pressures onreaction vessel 80, shown schematically in FIG. 4 and described in moredetail in FIG. 6 below, to effect the conversion of graphite to diamondin reaction vessel 80. The second end of piston 46 projects out ofreaction chamber 44 and is contacted by piston pusher 52, asdescribed'above. 1

Binding rings 82 and 84 are provided around piston 46. Binding ring 82always stays around the second end of piston 46 and binding ring 84 isfriction fitted around the body of piston 46 and slides along piston 46as the piston 46 traverses into reaction chamber 44. The function ofbinding rings 82 and 84 is described in detail in connection with thedescription of FIG. 5 below.

.FIG. 4 also shows electrical insulating member 86 positioned betweenupper end load. plate 48 and high pressure plate 42. Insulating member86 is provided so that reaction vessel 80 may be electrically heated byapplying a suitable potential between upper endv load plate 48 andpiston pusher 52. More details of the electrical heating manner areshown in connection with FIG. 6 below.

i As was mentioned above, both cylinder 46 and pressure vessel '72 are"preferably made from cemented tungsten carbide, which is one of thehardest materials presently known. However, even tungsten carbide hasdifficulty withstanding the pressures of the magnitude shown in thephase diagram of FIG 1. Pressure vessel 72 can be reinforced withinterference. fitting steel rings, as is shown in FIG. 4, but obviouslythis approach cannot be taken with piston 46, since it must be free to'move into reaction chamber 44. Thus, piston46 itself must withstandpressures in the magnitude of 50 to kilobars, or some means must beemployed to enable'it to do so. The best available tungsten carbidepistons are claimed by their manufacturer to be able to withstand apressure of only about 47 kilobars, and the pistons so obtainableusually do rupture when subjected to pressures of this order. Thus, itis seen that some means must be employed to strengthen the pistons towithstand substantially higher pressures while still allowing them totraverse further into reaction chamber 44 when the graphite to diamondconversion occurs to compensate for the resultant decreasein volume- Oneapproach which has proven to be satisfactoryis to provide binding rings82. and 84 on piston 46. Asis Piston 46 fits into the lower end ofreactionchamber l 44 in high pressure plate 42. Again, it is desirableto shown inFIG. 4, binding ring 82 is positioned with an interferencefit around the second end of piston 46 adjacent to piston pusher 52.Binding ring 84 is positioned with a friction fit'around the body ofpiston 46 and is initially placed near the top of piston 46. As piston46 enters reaction chamber 44, binding ring 84 is restrained by thebottom of high pressure plate 42 and slides along the side walls ofpiston 46 as it enters reaction chamber 44.

FIG. 5'shows graphically the effect of binding rings 82 and 84 on thecrushing strength ofa cemented tungsten carbide piston 46. The curvetherein plots the crushing strength in kilobars as a function of theratio of the unsupported length to the diameter of the piston.

Unsupported length means the length between binding rings 82 and 84. Itis seen that the use of the binding rings dramatically increases thecrushing strength of the piston and if binding rings are employed toprovide an effective length to diameter ratio of the unsupported pistonof one or less, a crushing strength in excess of 60 kilobars can easilybe obtained. This enables such apparatus to be used in diamondconversion to overcome the defects of the belt apparatus of FIG. 2.

Prior reports on the crushing strength of cemented tungsten carbidepistons can befound in an article by Kennedy and I-Iaygarth entitledCrushing Strength of Cemented Tungsten Carbide Pistons appearing in Thecylinder 84 which is made from Pyrex glass. An inner graphite sleeve 86is provided which is the electrical heater element for heating thecharge to the desired high temperature. Each end of reaction vessel 80is closed by an end plug 88, which is electrically conductive and whichserves as an electrical lead to graphite heater 86. Insulator rings 90are provided to prevent end plugs 88 from shorting out against the sidewalls of reaction chamber 44.

During the experiments which led to the present invention, it wasobserved that, for reasons not yet understood, the presence of hydrogenin reaction chamber 44 is very detrimental to the conversion of graphiteinto diamond. For this reason, the outer cylinder 82 of reaction vessel80 is constructed from sodium chloride, or pressed salt, or any othersuitable anhydrous material, even though this material is harder tohandle than a material such as tale. The problem with talc is that it isMgO.SiO,.H=.O. When the tale is subjected to the high temperatures andpressures within reaction chamber 44, the water of crystallization isbroken apart and the water is broken down into its hydrogen and oxygencomponents, thus liberating hydrogen into the reaction chamber. Forunknown reasons, this hydrogen so liberated greatly impedes theformation of diamonds.

The term anhydrogenous may be used to describe a material which containsno hydrogen in its composition, either as free hydrogen or as an elementin a 12 hydrogen-containing compound. Using the term so defined, it hasbeen found that reaction vessel should be formed entirelyfromanhydrogenous material.

Also, in order to maintain a hydrogen free reaction chamber, it wasdiscovered that hydrogen-containing lubricants should not be used in thereaction chamber area. In the piston-cylinder apparatus of FIGS. 3 and 4it is necessary to provide as close a tolerance as possible between theouter diameter of piston 46 and the inner diameter of reaction chamber44. In this general type of cylinder-piston high pressure apparatus, inorder to reduce the friction between the piston and the cylinder, it iscustomary to lubricate it with a lubricant such as molybdenum disulfide.This lubricant itself does not contain hydrogen, but it is usuallydissolved in a hydrocarbon grease to be coated on the walls. It has beenfound that greatly improvedresults are obtained if the molybdenumdisulfide is instead dissolved in a fluorocarbon base such asperflourated kerosine before being coated on the walls of the piston andcylinder. Thus, all materials present in the reaction chamber areselected to be anhydrogenous. e

The apparatus used in accordance with the present invention to convertgraphite to diamond has thus been described in detail; Consider now themanner in which this apparatus is operatedto effect the conversion. Amixtureof graphite and of the material to be used as a solvent is placedin reaction vessel 80 of FIG. 6. For example, the solvent may be anycarbon solvent listed in Hansen, Constitution of Binary Alloys,McGraw-I-Iill Book Co. (1958), such as the iron-nickel alloy discussedabove, and the proportions of the solvent to graphite may be 20 to 80percent. This mixture, termed the charge, is placed inside reactionvessel 80 and the end plugs 88 are placed upon it. Reaction vessel 8 0is then placed in reaction chamber'44 of the apparatus 40 of FIGS. 3 and4 and piston 46 is inserted into reaction chamber 44 until reactionvessel 80 is tightly confined between upper end load plate 48 and oneend of piston 46..The charge is then heated by applying an electricalpotential between upper end load plate 48 and piston pusher 52, thussupplying electrical current to graphite heater 86 through end plugs 88of reaction vessel 80. The hydraulic rams 54 and 62 are then actuated.The charge within reaction vessel 80 is thus heated and subjected tohigh pressures.

Continuing the description of the operation of the apparatus inaccordance with the invention, reference is now made to FIG. 7, which isa pressure-temperature phase diagram of carbon similar to that shown inFIG. 1. Shown therein is the equilibriumline l and the phase boundaryline 2, as was described in detail in connection with the description ofFIG. 1 above. Again, the curve 5-3-6 defines the diamond growing region4.

Pressure and temperature are applied to the charge in the reactionvessel 80 in the manner described above until a point 92 is reachedwhich lies-to the right of phase boundary line 2 but beneath equilibriumline 1. At this point the solvent isin its liquid phase and the graphiteis dissolved in the solvent. However, since point 92 lies belowequilibrium line 1, the carbon remains in the graphite state. Pressureis now increased by further actuating hydraulic ram 54 until the point94 is reached just above equilibrium line 1 in diamond mo ma propriatefor converting graphite into diamond and such a conversion occurs.

It has been found that by placing point 94 just above equilibrium line1, only a few relatively large diamonds will be formed from the chargerather than a large number of relatively small diamonds. Dramaticallyimproved results are obtained if the pressure at point 94 is limited tono more than 0.1 kilobar above equilibrium line 1. It is believed thatthe reason for this is that at a point quite near or practically onequilibrium line l,initial growth begins quite slowly and a small numberof diamonds begin crystallizing. As'these conditions are maintained fora period of time, additional graphite is converted todiamond, and thisadditional conversion growing region 4. At this point, the conditionsareapoccurs aroundthe initially formed diamonds, which may be thought ofas seeds. If these conditions are maintained'for a long enough period oftime, practically all of the graphite will be converted to diamond and asmall number of seeds will grow into relatively large diamonds. inpractice, diamonds of over one carat have been made in this manner. a

In contrast to this, if the pressure is increased to a point far intothe diamond growing region 4, many small diamonds are formed because theconditions are so far into the diamond growing region that diamondconversion occurs simultaneously all through the charge, rather than inonly a few locations at first, and thus the seed effect described abovedoes not occur.

Continuing now the description of theoperation of the presentinvention,-as diamondconversion occurs at point 94, the volume of thecharge decreases abruptly, because of the above mentioned increase indensity of the carbon as conversion occurs. To compensate for this,piston 46 is advanced ,further into reaction chamber 44 to reduce itsvolume while maintaining the 7 same pressure. It is primarily thisfeature'and the ability to effect this result that provides the greatadvances of this invention over the prior art. I

After the chargehas beenmaintained at the point 94 for a sufficientperiod of time, typically about 30 minutes for a charge of about 10grams, the temperature is then reduced by reducing the current throughthe electrical heater while maintaining the pressure at substantiallythe same level until the point 96 is reached which is to the left ofphase boundary line 2. This sequence is followed in order that thediamonds are not reconverted back to graphite. After the point 96 isreached the temperature and pressure may be reduced to room temperatureand pressure and the now diamond bearing charge withdrawn from thereaction vessel.

The following are theydetails of specific runs in which graphite wassuccessfully converted to diamond in the manner described above. It isnoted that each of the following examples indicates the percentage ofthe charge of graphite which was converted to diamond. In each instance,the diamond so converted was not separated from the solvent metal orfrom the unconverted graphite which might have remained in the charge.lnstead,the percentage of diamond conversion was calculated by measuringthe reduction in the volume of the charge, taking into account thehigher density of diamond described above.

EXAMPLE 1 A charge comprising 2.1 grams of graphite and 2.4 grams of asolvent which was an alloy containing 60 percent iron and percent nickelwas placed in the reaction vessel. The reaction vessel was placed in thereaction chamber, in which thepressure was increased to 54 kilobars andthe temperature increased to 1400 C. These conditions were maintainedfor 55 minutes, during which time period the piston was continuouslyadvanced into the reaction chamber while maintaining the pressure of 54kilobars to compensate for the volume reduction describedabove. At theend of this time period, temperature and pressure conditions werereturned to normal, in the manner described above. 80 percent of thegraphite was convertedto diamond.

EXAMPLE 2 A charge comprising 2.1 grams of graphite and-2.4 grams of asolvent which was an alloy containing 60 percent iron and 40 percentnickel was placed in the reaction vessel. The reaction vessel was'placedin the reaction chamber, in which the pressure was increased to62kilobars and the temperature increased to 1460 C. These conditions weremaintained for 22 minutes, during which time period the piston wascontinuously advanced into' the reaction chamber while maintaining thepressure of 62 kilobars, to compensate for the volume reductiondescribed above. At the end of this time period, temperature andpressure conditions were returned to normal in the manner describedabove. percent of the graphite was converted to diamond EXAMPLE 3 Acharge comprising 2.1 grams of graphite and 2.4

. grams of a solvent which was an alloy containing 60 percent iron and40 percent nickel was placed in the reaction vessel. The reaction vesselwas plac'edin the reaction chamber, in which the pressure was increasedto 58 kilobars and the temperature increased t0 1400 C. These conditionswere maintained for 12 minutes, during which time period, the piston wascontinuously advanced into the reaction chamber while maintaining thepressure of 58 kilobars to compensate for the volume reduction describedabove. At the end of this time period, temperature and pressureconditions were returned to normal, in the manner describedabove. .55percent of the graphite was converted to diamond.

EXAMPLE 4 A charge comprising 2.1 grams of graphite and 2.4 grams of asolvent which was an alloy containing 60 percent iron and40 percentnickel was placed in the reaction vessel. The reaction vessel was placedin the reaction chamber, in which the pressure was increased to 56kilobars and the temperature increased to 1460" C. These conditions weremaintained for 40 minutes, during which time period, the piston wascontinuously advanced into the reaction chamber while maintaining thepressure of 56 kilobars to compensate for the volume reduction describedabove. At the end of this time period, temperature and pressureconditions were returned to normal, in the manner described above. 45percent of the graphite was converted to diamond.

15 "EXAMPLE A charge comprising 2.1 grams of graphite and 2.4 grams of asolvent which was an alloy containing 60 percent iron and 40 percentnickel was placed in the reaction vessel. The reaction vessel was placedin the reaction chamber, in which'the pressure was increased to54'kilobars and the temperature increased to 1350 C. These conditionswere maintained for ll minutes, during which time period, the piston wascontinuously advanced into the reaction chamber while maintaining thepressure of 54 kilobars to compensate for the volume reduction describedabove. At the end of this time period, temperature and pressureconditions were returned to normal, in the manner described above. 60percent of the graphite was converted to diamond.

EXAMPLE charge'comprising "1.45 gramsof graphite and 1.45 grams of asolvent which was an alloy containing 78 percent nickel, percentchromium and 7 percent iron was placed in the reaction vessel. Thereaction ves- EXAMPLE 7 c Q A charge comprising 1.45 grams of graphiteand 1.45

v grams of a solvent which was an alloy'containing 78 percent nickel, 15percent chromium and 7 percent iron, was placed on "the reaction vessel.The reaction vessel was placed in the reaction chamber, in which thepressure was increased to 57 kilobars and the temperature increased to1250 C. These conditions were maintained 18 minutes, during which timeperiod, the piston was continuously advanced into thereaction chamberwhile maintaining the pressure of 57 kilobars to compensate for thevolume reduction described above. At

the end of this time period, temperature and pressure conditions werereturned to normal, in the manner described above. 40 percent of thegraphite was converted to diamond. 1

EXAMPLE 8 4 piston was continuously advanced into the reaction chamberwhile maintaining the pressure of 60 kilobars to compensate for thevolume reduction described above. At the end of this time periodtemperature and EXAMPLE 9 A charge comprising 2.1 grams of graphite and2.4 grams of a solvent which was an alloy containing 60 percent iron and40 percent nickel, was placed in the reaction vessel. The reactionvessel was placed in the reaction chamber in which the pressure wasincreased to 56 kilobars and the temperature increased to 1340 C. Theseconditions were maintained for minutes, during which time period, thepiston was continuously advanced into the reaction chamber whilemaintaining the pressure of 56 kilobars to compensate for the volumereduction described above. At the end of this' time period, temperatureand pressure conditions were returned to normal, in the manner describedabove. percent of the graphite was converted to'diamonds While theinvention is thus disclosed and the preferred embodiment described indetail, it isnot intended that the invention be limited to this shownembodiment. Instead, many modifications willoccur to those skilled inthe art which lie within the spirit and scope of the invention.Accordingly, it is intended that the invention be limited only by theappended claims.

' What is claimed is: I I

1. A machine for converting graphite to diamond, comprising incombination: i

a body formed from high pressure resistantmaterial having a cylindricalcavity therein which forms a cylindrical reaction chamber having apredetermined diameter,

a cylindrical reaction vessel positioned in said chamber'for holding acharge of graphite to be converted to diamond; said reaction vesselhaving an outside diameter such that it has a snug sliding fit with saidreaction chamber, I

a piston having a cross-section substantially corresponding to thecross-section of said cylindrical reaction chamber and having afixed-length and fixed constant diameter positioned in said reactionchamber, the fixed diameter of said piston being substantially equal tothe outside diameter of said reaction vessel for a distance'along saidpiston sufficient to allow said piston to advance further into saidreaction chamber to maintain constant pressure on said reaction vesselduring the time that said graphite is being converted to diamond, said2. The combination of claim 1 in which said means for maintaining theratio of the unsupported length to the diameter of said piston at avalue less than the ratio of the fixed length to the fixed diameter ofsaid piston comprises a first binding ring positioned around said secondend of said piston and a second binding ring positioned around the bodyof said piston.

within I substantially equal to the inner diameter of said reactionchamber and having a' length shorter than the length of said reactionchamber, an electrically conductive sleeve positioned concentricallywithin said cylinder and first and second electrically conductive endcaps closing the ends of said cylinder and making electrical contactwith said sleeve.

6. The combination of claim in which said cylinder is formed from ananhydrogenous material and said sleeve is form ed from graphite.

7. The combination of claim 6 which further includes means for heatingsaid reaction vessel, comprising means for applying an electricalpotential between said first and second end caps.

8. The combination of claim 7 in which said means for maintaining theratio of the unsupported length to the diameter of said piston at avalue less than the ratio of the fixed length to the fixed diameter ofsaid piston comprises a first binding ring positioned around said secondend of said piston and a second binding ring positioned around the bodyof said piston.

9. The combination of claim 8 in which said secondbinding ring ispositioned around the body of said piston with a sliding friction fit sothat said second binding ring slides along the body of said piston assaid piston traverses further into said reaction chamber.

10. The combination of claim 9 in which the ratio of the unsupportedlength to the diameter of said piston is maintained at a value of lessthan one.

1. A machine for converting graphite to diamond, comprising incombination: a body formed from high pressure resistant material havinga cylindrical cavity therein which forms a cylindrical reaction chamberhaving a predetermined diameter, a cylindrical reaction vesselpositioned in said chamber for holding a charge of graphite to beconverted to diamond, said reaction vessel having an outside diametersuch that it has a snug sliding fit with said reaction chamber, a pistonhaving a cross-section substantially corresponding to the cross-sectionof said cylindrical reaction chamber and having a fixed length and fixedconstant diameter positioned in said reaction chamber, the fixeddiameter of said piston being substantially equal to the outsidediameter of said reaction vessel for a distance along said pistonsufficient to allow said piston to advance further into said reactionchamber to maintain constant pressure on said reaction vessel during thetime that said graphite is being converted to diamond, said piston beingpositioned so that its first end can exert force against said reactionvessel and its second end extends out of said reaction chamber, meansfor applying a force to said second end of said piston, therebygenerating high pressure within said reaction chamber, and means formaintaining the ratio of the unsupported length to the diameter of saidpiston at a value less than the ratio of the fixed length to the fixeddiameter of said piston.
 2. The combination of claim 1 in which saidmeans for maintaining the ratio of the unsupported length to thediameter of said piston at a value less than the ratio of the fixedlength to the fixed diameter of said piston comprises a first bindingring positioned around said second end of said piston and a secondbinding ring positioned around the body of said piston.
 3. Thecombination of claim 2 in which said second binding ring is positionedaround the body of said piston with a sliding friction fit so that saidsecond binding ring slides along the body of said piston as said pistontraverses further into said reaction chamber.
 4. The combination ofclaim 1 in which the ratio of the unsupported length to the diameter ofsaid piston is maintained at a value of less than one.
 5. Thecombination of claim 1 in which said reaction vessel comprises acylinder having an outer diameter substantiaLly equal to the innerdiameter of said reaction chamber and having a length shorter than thelength of said reaction chamber, an electrically conductive sleevepositioned concentrically within said cylinder and first and secondelectrically conductive end caps closing the ends of said cylinder andmaking electrical contact with said sleeve.
 6. The combination of claim5 in which said cylinder is formed from an anhydrogenous material andsaid sleeve is formed from graphite.
 7. The combination of claim 6 whichfurther includes means for heating said reaction vessel, comprisingmeans for applying an electrical potential between said first and secondend caps.
 8. The combination of claim 7 in which said means formaintaining the ratio of the unsupported length to the diameter of saidpiston at a value less than the ratio of the fixed length to the fixeddiameter of said piston comprises a first binding ring positioned aroundsaid second end of said piston and a second binding ring positionedaround the body of said piston.
 9. The combination of claim 8 in whichsaid second binding ring is positioned around the body of said pistonwith a sliding friction fit so that said second binding ring slidesalong the body of said piston as said piston traverses further into saidreaction chamber.
 10. The combination of claim 9 in which the ratio ofthe unsupported length to the diameter of said piston is maintained at avalue of less than one.